It has been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond 20 MHz will be based on the aggregation of a plurality of component carriers at different frequencies. Such carrier aggregation can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at 800 MHz, 2 GHz, and 3.5 GHz). Whilst a legacy mobile device may only be able to communicate using a single, backward compatible, component carrier, a more advanced multi-carrier capable terminal would be able to simultaneously use the multiple component carriers.
As mobile (cellular) communication technology has developed there have been proposals to provide enhanced communication in relatively small geographic regions by having small cells (e.g. ‘pico’ or ‘femto’ cells) that coexist with a larger (‘macro’) cell and provide enhanced communication capabilities in the localised geographic region that the small cell covers. These small cells can be provided on the same carrier as the macro cell or can be provided on a different (e.g. higher frequency) dedicated carrier.
More recently, it has been proposed to allow user data for a particular user device such as a mobile telephone or other mobile communication device (also referred to as ‘user equipment’ or a ‘UE’) to be communicated via a different cell to the cell via which control data for that user device is communicated. Specifically, it has been proposed to allow the user plane (U-plane) and control plane (C-plane) for a particular user device to be split between the small cell and the macro cell such that U-plane data is communicated via the small cell and C-plane data is communicated via the macro cell.
The small cell of this proposal is, effectively a ‘pseudo’ cell or ‘phantom’ cell because it does not provide conventional cell-specific signals and/or channels such as carrier reference signals, master information/system information broadcasts, primary/secondary synchronisation signals, etc.
In theory, the C-plane/U-plane split of this proposal provides an optimisation of: the benefits of the better connectivity typically offered by a macro cell for critical control signalling; and the benefits of higher throughput and more flexible, energy efficient, and cost effective communication offered by a small cell using a higher and/or wider frequency band for higher volume user data.
However, the C-plane/U-plane split proposal presents a number of challenges that need to be addressed if such a proposal is to be implemented practically in the global communication network.
One such challenge is the provision of appropriate communication security where different base stations are responsible for U-plane signalling and C-plane signalling respectively whilst ensuring that the user device is able to encipher/decipher user data and control data correctly. This has the potential to add significant unwanted complexity to signalling between the core network, the base station, and the user device.
Moreover, in order to ensure appropriate security it is beneficial to be able to, from time to time, regenerate the security keys used for encryption and integrity protection (‘re-keying’ or ‘key-refresh’). Such dynamic key changing can be the result of explicit re-keying or implicit key-refresh procedures. To ensure that the security parameters used for ciphering and integrity protection remain unique, for example, key refresh is typically required when the Packet Data Convergence Protocol (PDCP) counter (‘PDCP COUNT’), which is used as a ciphering input, reaches its limit and ‘wraps around’ or ‘rolls over’ back to its starting value. Re-keying/key-refresh avoids the risk that previously used PDCP COUNT values are re-used, in combination with the same security key, as inputs for ciphering thereby avoiding the cyclic re-use of earlier security parameters.
However, currently, such dynamic key refreshing is not possible when the U-plane and C-plane are split because the PDCP count is maintained in the U-plane whilst the control signalling required for re-keying occurs in the C-plane.